This invention relates to heat exchanger assemblies and more particularly to an improved fin array design for use in a variety of heat exchanger assemblies and a method of making the fin array.
FIG. 1 illustrates a prior art heat exchanger assembly in the form of a condenser typically used in air conditioning units for vehicles. The heat exchanger assembly 10 includes a pair of opposed, spaced, generally parallel headers 11 and 12. The headers 11 and 12 each define a series of generally parallel slots or openings 13 for receiving the ends 14a and 14b of tubes 14 that extend in fluid communication between the headers 11 and 12. Each of the headers 11 and 12 includes a fitting 15 and a cap 16. The fittings 15 operate as either an inlet or outlet for circulation of fluid through the headers 11 and 12 and tubes 14. The fittings 15 can be operatively connected, such as by tube 17 or other appropriate tubing, to a heat exchanger system such as for a air conditioning unit for a vehicle. The heat exchanger assembly 10 also includes channels or flanges 18 and 19 in order to provide rigidity to the structure.
A plurality of elongated serpentine fins 20 extend between the headers 11 and 12 along each of the heat exchanger tubes 14. Each of the fins 20 follows a serpentine pattern and has rounded crests that are alternately connected to the top and bottom tubes 14 by a process such as brazing.
It is well known in the art that the efficiency of a heat exchanger assembly is mainly limited by the heat flux between the fins and the ambient air, which receives the heat from the system or transmits heat into the system depending upon the application. For example, in the case of mechanical refrigeration systems, it is known that the heat flux per unit of area between the tube walls and refrigerant or between the tube walls and fins is very high relative to the heat flux per unit area between the surrounding air and the fin and tube surfaces. It is also known in the art that the portion of the fin that first cuts through the air has the highest heat flux per unit area.
To improve heat flux between the fins and the ambient air, many heat transfer systems employ a fan to move more air per unit of time across the fins. As another example, moving vehicles such as automobiles typically position the air conditioning condenser on the front of the car to provide maximum air flow across the fin and tube surfaces.
In another system to improve heat flux between the fins and ambient air, the fins are manufactured to include small louvers in each fin that catch the air and force the air to flow past or over the heated or cooled fin surfaces. A fin array 21 including louvers on the fins is shown in the prior art fin assembly of FIG. 2. The fin array 21 is folded in a serpentine pattern to form a series of alternating upper and lower crests 22 and a plurality of individual fins 23. Each of the individual fins 23 includes a plurality of louvers 24.
The elongated fin array 21 is typically manufactured from strips of metal, such as copper or aluminum, that are run through rotary cutting dies that shape the openings in a strip, shape the louvers by pushing them inward or outward from the strip, and then fold the fins using a "star wheel" style roller which imparts a rounded bend to the fin stock. The fin array 21 including louvers 24 on the fins 23 improves the heat flux as compared to traditional non-louvered fins. However, the louvered fins are less than optimal for maximizing heat flux between the fins and ambient air and are difficult and expensive to manufacture.
For example, the louvers 24 on the fins 23 do not extend across the entire length of the individual fins due to the rounded bend area at crests 22 and thus form bypass passageways labeled 25 in FIG. 2. Air can thus pass entirely through the fins 22 at bypass portions 25 without encountering the louvers 24 or substantially contacting the fins 23.
In the louvered fin array 21, the louvers 24 are also aligned directly behind each other such that the air tends substantially to contact only the first row or two of the louvers 24. Thus, the louvers 24 toward the back of the fin set do not "see" fresh air since they are in the shadow of the first louvers.
The louvered fin array shown in FIG. 2 is typically manufactured by cutting the fins in a traditional shearing die technique. With most metals such as copper or aluminum, those skilled in the art know that large amounts of lubrication are required for shear cutting of the material in order to prevent heat build-up in the cutting tools. However, the lubricating oils must be substantially removed from the fins after the cutting process so that the fins are clean for brazing the fins to the tubes. The process of removing the lubricating oils from the fins is an extremely expensive process and results in environmentally dangerous by-products.
This manufacturing process also commonly results in relatively large fin height variations that can lead to poor bonding between the fins and tubes. As a consequence of tolerance build up, added to run by run in the full assembly process, the rounded upper and lower crests of the fin array may not allow for complete fin to tube contact if the tubes are thinner than normal or if the fins have been folded with too small of a height. Poor bonding between the fins and tubes can dramatically decrease the efficiency of the entire heat exchanger assembly. If, on the other hand, fins have been folded with too great a height and/or tubes are thicker than normal, then some runs of the fins may be crushed out of shape allowing increased (or decreased) by-pass (or breakage). Both of which are detrimental to heat transfer.
In the prior art manufacturing processes, it is well known to apply pressure to the stacked fins and tubes during the assembly process to ensure adequate contact between the fins and tubes prior to brazing. However, the fins must have sufficient physical vertical strength so that the fins will not collapse under this pressure. Typically, the fins in such heat exchanger assemblies are straight or flat along their horizontal width or such cross section, and such a flat structure only provides a minimum column strength to the structure.